This proposal describes research on the bases of developmental dyslexia, using behavioral studies of normal and impaired readers and computational modeling to identify factors that give rise to word processing deficits. There is considerable uncertainty regarding the bases of dyslexia; existing evidence implicates several factors including phonological, visual-perceptual, and auditory perceptual impairments. The proposal describes behavioral studies that will examine components of word reading skill with the goal of identifying determining whether different types of impairments contribute to the deficits in word level processing that are commonly observed in dyslexia. Word recognition is a complex skill drawing on several types of knowledge, processes, and cognitive capacities. Failures to acquire age-appropriate reading skills could derive from impairments in any of these areas. The theoretical framework provided by connectionist models is used as a basis for generating predictions concerning these different sources of dyslexic impairment. This framework has yielded explicit computational models of word processing. It also provides a powerful way to examine the bases of impaired behavior: they can be simulated by introducing various types of anomalies in a working model of normal performance. This permits us to identify different sources of reading impairment and generate predictions concerning their effects on the reading and related tasks used in the experimental studies. A four- year longitudinal study of dyslexic readers in grades 3-4 will be conducted using a large battery of measures to assess orthographic, phonological, visual- and auditory-perceptual and other components of reading. Comparison groups ~f normal readers matched on age and word recognition skill will also be followed for 4 years. The data will be used to generate subject profiles indicating deficit areas for each individual, and the stability and predictive validity of their behavioral profiles overtime will be assessed. Additional experiments examining key component skills in more depth will be conducted during off-months for the longitudinal study. These experiments will focus on (a) putative deficits in speech perception associated with dyslexia and (b) the effects of word- level variables (such as frcquency, spelling-sound regularity and imageability) on word and non-word naming among dyslexic and normal readers. The goal of the simulations is understand how specific impairments in the lexical system give rise to the typed of impaired behavior observed in experimental studies. The behavioral data will be related to connectionist models that simulate detailed aspects of normal and atypical development. The models' patterns of errors on different word types will be compared to the errors and reaction times of dyslexic children on these word types as a test of the validity of the theoretical model.